1. Field of the Invention
This invention relates to air-based catheter systems for monitoring pressure in a mammalian body especially as applied to measuring intracranial pressure (ICP) in the brain.
2. Description of the Prior Art
An air-based catheter consists of a catheter with an air lumen that communicates with a bladder at or near its distal end and with a connector at or near its proximal end. The bladder volume of the catheter changes as pressure changes in accordance with P1V1=P2V2 and thereby causes the pressure of the gas within the catheter to equal that of the external environment surrounding the bladder. The media in which the bladder senses pressure must be able to move toward or away from the bladder to cause the requisite change in bladder volume. Movement of the media is not a problem when the bladder is immersed in a flowable liquid such as blood. There is a problem when brain tissue is the media in that brain tissue has a limited ability to move. An air-based catheter designed to measure ICP must limit the movement required of the bladder so that the distance the bladder wall moves as it portrays the wave form of a heart beat does not exceed the intrinsic ability of brain tissue to move.
The prior art system, one made by Spiegelberg Gmbh, has a long air line that connects the catheter bladder to a bedside instrument. The air line significantly increases total system volume and thereby increases the volume change required of the bladder to reflect a given pressure change. Spiegelberg addresses the limited ability of brain tissue to move by purposefully underfilling the bladder. The underfilled bladder approximates a flat profile when placed in the brain. The (x)(y) planar area created by the flattened profile of the bladder reduces the movement required of brain tissue in the z dimension and does so in direct proportion to the x and y dimensions. The planar area chosen is one that limits the wall movement required of the bladder to a value compatible with the ability of brain tissue to move. The planar area is defined by the potential volume of the bladder and the degree to which it is underfilled. Spiegelberg uses a 425-ul bladder filled about 25% full. The underfilled bladder makes it possible for the Spiegelberg air-based catheter to measure pressure in the brain. An electromechanical pump is used to address the limited operating life of a bladder with a large surface area and small contained volume.
The volume of air in the bladder displaces brain tissue. It is therefore desirable that the air volume be as small as possible to limit tissue displacement. In the Spiegelberg system, most of the system volume that must be acted upon by the bladder is contained in the long air line that connects the intracranial catheter to a bedside instrument housing a pressure transducer. The connecting line represents about 80% of the total system volume. It is therefore responsible for 80% of the air volume required by the bladder to accomplish the required volume change. Bladder volume and therefore brain trauma in prior art are therefore largely defined by the volume of air in the air line that connects the catheter to a bedside instrument. Furthermore, the volume of air in the connecting air line largely defines the potential volume of the oversized bladder required to achieve the underfilled state necessary to limit bladder wall movement in the Z direction. The surface area of the oversized bladder increases air lost by diffusion and decreases the operating life of the catheter, i.e., the period of time after which air lost by diffusion must be replaced.
Brain trauma caused by the air volume within the bladder can be minimized if the air in the system outside the bladder is held to a minimum. If Vnb=volume of non-bladder air and Vb=volume of bladder air, then system volume=(Vnb+Vb). As system volume changes from V1 to V2, V1xe2x88x92V2=(Vnb+Vb1)xe2x88x92(Vnb+Vb2). Vb is the only variable volume; therefore the change required of the bladder is least when the non-bladder air volume (Vnb) is zero. Accordingly, a design least demanding in terms of both bladder wall movement and volume of brain tissue displaced is one wherein the ratio of (system volume)÷(bladder volume) approaches 1:1. The ratio in the Spiegelberg device is 5:1, a ratio far from ideal. The 5:1 ratio calculation includes 550 ul of gas contained in the 154 cm long air line and approximately 125 ul of gas in the bladder for a total system volume of about 675 ul.
The patents and literature covering gas column pressure monitoring catheters describe systems designed for various pressure measurement applications. Catheters constructed according to these teachings, however, are not optimal for use in measuring intracranial pressure (ICP) in that they do not minimize brain trauma nor do away with the need for an electromechanical pump. The deficiencies, if addressed, would greatly improve the usefulness and safety of the catheter.
A number of prior art devices transmit physiological pressure through a gaseous medium. The following United Sates and foreign patents/patent publications have described pressure measuring catheters and other pressure transmitting systems wherein a gas is utilized as a pressure-transmitting medium in at least a portion of the system: U.S. Pat. No. 5,573,007 (Bobo), U.S. Pat. No. 2,840,069 (Squire et al), U.S. Pat. No. 4,227,420 (Lamadrid), U.S. Pat. No. 4,300,571 (Waldbilling), U.S. Pat. No. 4,314,480 (Becker), U.S. Pat. No. 4,648,406 (Miller), U.S. Pat. No. 4,841,984 (Armeniades et al.), U.S. Pat. No. 5,105,820 (Moriuchi, et al.), Patent publications: WO82/02657 (Ebert), WO86/03957 (Spiegelberg), WO90/11717 (Utah Medical Products, Inc.) Prior art pertaining specifically to a gas-column pressure-monitoring catheter that employs a flaccid bladder is explicitly covered in WO86/03957 (Spiegelberg) and U.S. Pat. No. 5,573,007 (Bobo).
The only prior art air-based catheter capable of measuring ICP in brain tissue is a device produced by Spiegelberg Gmbh in Germany. The Spiegelberg device addresses the matter of controlling bladder wall movement by using a partially filled bladder. The bladder has a potential volume of 425 ul net of the volume occupied by the catheter within. Once the bladder is inserted into the brain, an electromechanical pump defines ICP level. The pump then injects an amount of air into the bladder that ranges from 50 to 100 ul. The electromechanical pump deflates and refills the bladder on an hourly basis. The terms injected or filled as used hereinafter means the volume of gas inserted into the bladder where the pressure of the injected gas is equal to atmospheric pressure. The total air volume in the bladder consists of the volume in the uncollapsed portion of the bladder prior to air injection of about 50 ul plus the injected volume of 50-100 ul. The air volume of the bladder is therefore about 125 ul. The total volume of air in the bladder defines the brain tissue pushed aside and is correlated to the trauma imposed upon the brain.
The partially filled bladder assumes a flattened shape when positioned in the brain. If the area of the flattened bladder is described as (x)(y) and the movement of opposite walls of the bladder described as z, then z=the volume change divided by the area or (V1xe2x88x92V2)÷(x)(y). Z decreases as (x)(y) increases. The flat partially filled bladder used by the Spiegelberg device limits bladder movement by using a (x)(y) value such that z falls within the brain""s ability to move.
The percentage the bladder must be underfilled to function in brain defines the absolute size of the bladder, the bladder surface area and in turn, the rate at which air is lost by diffusion. Air loss defines the system""s operating life, i.e., the time the bladder can function before air lost by diffusion must be replaced. The fact that the Spiegelberg bladder must be substantially underfilled to function increases surface area and decreases operating life. The operating life of the Spiegelberg device is further reduced by the relatively inefficient type of bladder used. The Spiegelberg device uses a sleeve-like bladder open at both ends. The bladder is placed along the axis of the catheter body and the bladder ends joined to the catheter. The sleeve-like bladder used is relatively inefficient for several reasons. First, the presence of the catheter body within the bladder reduces the bladder""s usable air volume. Second, as the bladder nears exhaustion, the catheter body holds the opposing walls of the bladder apart in a tent-like fashion and renders part of the bladder volume useless. Both effects reduce collapsing efficiency and thereby reduce operating life.
Operating life is also decreased by an additional effect of the large gas volume in the air line that connects the bladder with the bedside transducer. The volume, essentially dead space, accounts for 80% of the bladder volume change required to respond to intracranial pressure fluctuations. Hereinafter, the change in bladder volume caused by pressure change will be referred to as the stroke of the bladder and can be visualized as the z component of (x)(y)(z) in the previously discussed formulae. The stroke of the bladder would be reduced by 80% if the dead space component of the air line could be eliminated from total system volume. A smaller stroke, i.e. a smaller delta z value, would allow the bladder to use more of the bladder air before z=0. A reduction in dead space would thereby increase operating life by reducing bladder stroke. Spiegelberg deals with the limited operating life of the bladder by use of an electromechanical pump that deflates and inflates the bladder at hourly intervals.
The use of a pump to vent and replace air in the bladder on an hourly basis provides another function, that of purging water vapor from the system. The water vapor diffusing into the bladder over the days or weeks that an ICP catheter may be in service can be substantial. If the air in the bladder becomes saturated with water vapor, the regular pulsing of the bladder will cause water vapor to condense and accumulate in the bladder. The condensate will at some time enter the catheter lumen, impede the flow of air and distort the pressure reading. The pump of the Spiegelberg system neutralizes the effect of water diffusion by flushing the bladder each hour with room air, thereby replacing moist air with dry air.
Two deficiencies of the prior art have thus far been explored. First, the volume of air required by a bladder in a system with a long air line displaces an undesirable volume of brain tissue. Second, the egress of air and the ingress of water through the bladder wall are such that an electromechanical pump must be used to exchange and replace air on an hourly basis.
The prior art does not teach a means to lessen brain trauma by reducing non-bladder air volume in an ICP catheter to enable use of a small volume bladder. It particularly does not teach a construction wherein the air volume in the bladder can be limited to 35 ul or less and wherein such a small bladder can operate for more than an 8-12 hour nursing shift.
The prior art does hot teach a system wherein a bladder can function in brain tissue when filled to +50% of its potential volume. The Bobo 007 patent shows a long catheter with significant dead space. The bladder resides in a normal ventricle filled with cerebral spinal fluid (CSF). It will not work if surrounded by brain tissue. The prior art catheter currently marketed by Spiegelberg Gmbh uses a large bladder that is filled to about 25% of its potential volume. It cannot measure ICP if fully filled. The +50% filled bladder of the present invention incorporates more air for a given bladder size and proportionally extends operating life.
No prior art device describes a tip-mounted bladder capable of operating in brain tissue. The prior art does not teach the use of a stent to preserve the elongated shape of a tip-mounted bladder as it enters brain tissue so it does not fold back over itself. It furthermore does not teach the use of a stent with a small cross section to minimize loss of useable air volume. Bobo 007 discloses a flaccid bladder mounted on the distal tip of the catheter that does not employ a stent. The bladder in 007 is used to measure pressure in body spaces wherein the bladder is immersed in liquid such as blood. Such a fluid permits free movement of the bladder and allows the bladder to assume its natural elongated shape without the assistance of an internal support element.
As regards extending operating life by reducing water ingress and air loss, no prior art device teaches constructing the bladder of an air-based catheter from rubber rather than plastic. Rubber, as used herein, refers to materials such as buna N, neoprene, butyl nitrile and butyl rubber. Rubber materials have extraordinarily low water permeability qualities, especially butyl rubber. For example, the water permeability of butyl rubber is {fraction (1/200)} that of polyurethane, the base material used in the Spiegelberg device. A butyl rubber bladder essentially blocks water ingress to the extent that a bladder can operate for extended periods without the need to purge humid air. Furthermore, prior art does not teach using an ultra thin butyl bladder to reduce air loss as a means of extending operating life while preserving pressure response. The Bobo 007 patent teaches the use of a variety of plastic materials including polyvinyl chloride, polyurethane, polyvinylidene and combinations thereof but does not teach the use of rubber in constructing a bladder. The use of butyl rubber vs. polyurethane significantly reduces air lost by diffusion. The extended life made possible by the use of a thin butyl membrane plays a critical role in feasibility of substituting a manual pump for the electromechanical pump now used
None of the prior art ICP devices separate the functions of pressure measurement and CSF drainage. The separation of functions allows measurement of ICP with a minimally invasive sensor prior to insertion of a more invasive CSF drainage catheter if drainage is required. The prior art system that can both measure pressure and drain CSF requires placement of a catheter in a ventricle deep within the brain at the onset of the procedure.
It is common practice to drain cerebral spinal fluid (CSF) to relieve pressure if the patient""s ICP exceeds 20 mmHg. The decision to drain CSF is based solely on whether or not the ICP is elevated. The existing art provides two options to care for a patient. The first option is minimally invasive. It can measure ICP but cannot drain CSF. The second option is more invasive and can both measure ICP and drain CSF. The first option involves placing a 3-5 Fr pressure sensor 1 cm deep into the brain. It has no drainage capability. This approach is good if ICP remains below 20 mmHg and drainage is not required, as it is minimally invasive. It is not good if ICP is elevated and drainage is required in that a second hole must be drilled in the skull to place a ventricular catheter in the brain. The second option involves placing a single lumen 9-10 Fr ventricular catheter into a ventricle located 6 cm deep within the brain. The catheter lumen is attached to a water filled line, which in turn is attached to an external pressure transducer that registers ICP. The water filled lumen can be used to drain CSF if ICP becomes elevated. This approach is good if drainage is required in that the drainage function is immediately available and only one hole need be drilled in the skull. It is not good if drainage of CSF is not required in that it is a more invasive way to monitor ICP than the first option. Presently used systems present a dilemma in that either option could subject the patient to unnecessary trauma. No prior art device provides the ability to look at ICP with a minimally invasive monitor as a first step and then, if need be, introduce a drainage catheter as a second step.
None of the prior art describes a bolt wherein the angle of entry of a drainage catheter can be varied over a wide range to target a ventricle not aligned with the axis of the bolt.
None of the prior art devices provide for the replacement of a clogged drainage catheter with a new catheter in a manner that reduces the risk of infection. Hospital protocol recommends against replacing a clogged drainage catheter with a new catheter in a currently used site due to the risk of infection. Protocol recommends that catheter replacement be done in a fresh site through a new hole in the patient""s skull.
None of the prior art devices explicitly address the risk of infection of bolt-mounted catheters. The literature reports that patients served by bolt-mounted catheters have a 4% chance of infection compared to 1% for patients served by a catheter tunneled beneath the scalp for several inches after it exits the drill hole. Prior art bolts do not provide an explicit means of intercepting bacteria en route to the drill hole nor provide a means of discouraging bacteria movement down the drill hole.
Co-pending patent application Ser. No. 09/379,282 by Bobo teaches mounting the transducer on a bolt affixed to the skull. Mounting the transducer proximal to the patient reduces the dead space volume at least an order of magnitude compared to the prior art system wherein the catheter uses a long air line to connect the catheter bladder to a bedside instrument. In the case of a catheter mounted in a bolt, the reduction in dead space reduces bladder wall movement to the extent that a nearly fully filled bladder can be used to measure ICP. In contrast, the dead space of Spiegelberg""s long air line requires that a substantially underfilled bladder be used to form an X-Y planar area large enough to make movement in the z dimension compatible with brain tissue.
Although the bladder of a bolt-mounted catheter can operate in a nearly fully filled condition, it would be impractical to design a device that will inject precisely the right amount of air to fully shape the bladder at 0 mmHg STP. The four reasons that prevent precise filling are listed in the order of their impact:
1. The residual air in the bladder before injection varies with how high the ICP is; more squeeze=less air in the bladder=more room for injected air. The maximum injected volume varies with ICP.
2. The temperature of the patient will affect V1 and V2 in the equation P1V1/T1=P2V2/T2.
3. The CO2 level in the brain tissue will result in an initial ingress of gas since CO2 diffuses in more rapidly than other gases present diffuse out.
4. The dimensions of the bladder cannot be precisely controlled. Therefore, the air injector must be designed to comprehend dimension tolerances.
None of prior art devices explicitly address the potential reading error caused by brain tissue as it resists dissection by an inserted bladder. At pressures under approximately 20 mmHg, the pressure inside the bladder is a composite of the intracranial pressure (ICP) and the resistance of the brain tissue surrounding the bladder to be dissected by the bladder. The dissection pressure component rises inversely with ICP and directly as the percentage inflation of the bladder with air increases. So, the lower the ICP and higher the percentage inflation, the greater the error. At some point, around 20 mmHg, the pressure in the bladder is such that it completely pushes aside brain tissue, at which time the dissection component is insignificant. In order to provide very accurate readings throughout the 0-20 mmHg range, the system is equipped with a modification that dissects the brain tissue in which the bladder will reside. The dissection component of the ICP reading is thereby rendered insignificant and the accuracy of the reading is preserved throughout the range. The inventors are unaware of any published information on the Spiegelberg device related to dissecting brain tissue. The device presumably employs some algorithm to determine exactly how much air should be injected into the bladder for a given ICP. Whether or not brain tissue is dissected by the algorithm and whether or not dissection is a planned or serendipitous event is unknown.
A first objective of the present invention is to minimize trauma inflicted upon brain tissue by minimizing the air volume required by the bladder of a gas-column catheter. Air volume is minimized by mounting the pressure transducer adjacent the patient. In one version of the present invention, the transducer is directly mounted on a bolt attached to the skull of the patient. In another version, the transducer is attached to the proximal end of a flexible catheter that is as short as possible. The minimal length is achieved by securing the proximal end near the patient""s neck or shoulder by adhesive tape or by a gown clip attached to the cable. Mounting the transducer adjacent to the patient greatly reduces the dead space in the long air line used in the prior art to connect the bladder to a bedside instrument housing a pressure transducer. Non-bladder volume, essentially dead space, is reduced a factor of approximately 110 in the case of a bolt-mounted transducer and a factor of 11 in the case of the flexible catheter. The reduction in dead space significantly reduces the volume of air required by the bladder to respond to pressure fluctuations by reducing the bladder stroke. A portion of the reduction in required bladder volume is added back to the bladder to assist in achieving a second objective, that of extending the operating life sufficiently to replace an electromechanical pump with a manual pump. The present invention, including air added to extend operating life, achieves a fourfold net reduction in bladder air volume and a fourfold reduction in brain tissue volume displaced, thereby achieving our first objective of reducing brain trauma.
The second objective of the present invention is the use of the manual pumping system described in Bobo 007 in lieu of the costly electromechanical pump required to support current air column catheters. This objective is of great economic importance in that a manual system eliminates the capital cost of an electromechanical system and thereby eliminates a significant disincentive to adopt an air-based ICP system. From a practical standpoint, it is important that maintenance of a manual system not be unduly burdensome to the nursing staff in terms of the frequency at which air must be replaced. One pumping event per shift would be compatible with the normal hospital protocol of checking proper functioning of a monitoring instrument. Five elements of the present invention contribute to the attainment of a once-per-shift maintenance schedule. Three are derivative of the reduction in dead space. First, the reduction in dead space reduces gas loss by diffusion by enabling the use of a small bladder with a correspondingly small surface area. Second, the reduced stroke of the bladder allows the bladder function in a more fully depleted state at end of its operating life. The smaller delta z extends the time before z=0. Third, the reduction in wall motion allows the bladder to function in brain tissue when filled to +50% of its potential volume vs. 25% as in the case in the prior art. The additional air volume extends operating life. The short flexible catheter function is filled to +50%. It functions in a partially filled state to decrease movement in the z dimension and thereby deal with the increase in dead space. The lesser air volume in the bladder of a flexible catheter results in a somewhat shorter operating life, but one consistent with the desired life of a bladder using a manual pump. Two other elements affect operating life. The sleeve bladder now used is replaced with a more efficient tip-mounted bladder, which allows the bladder to use more of the available air. Finally, use of a butyl rubber bladder extends operating life by reducing air loss by diffusion and preventing the ingress of water. The extension in operating life due to the combined effect of the five elements make it practical to replace an electromechanical pump with a manual pump.
A third objective of the present invention is to provide a bolt that allows a doctor to insert a small probe into the brain to measure ICP and then, if ICP is elevated, to insert a drainage catheter into a ventricle through a separate passageway in the bolt.
A fourth objective of the present invention is to provide a bolt-based system that allows the drainage catheter to be introduced into the brain at such an angle as may be required to target a ventricle. Since the hole drilled in the skull is done by a hand-held drill, the trajectory of the drill is not well controlled. Therefore, the bolt""s axis may or may not be aligned with a ventricle. Furthermore, the target ventricle may shift laterally toward or away from the midline of the skull as a result of trauma. Should it be necessary to insert a drainage catheter, the doctor must be able to vary the entry angle of the catheter in relation to the axis of the bolt to properly target the ventricle. The present invention allows the doctor to introduce a drainage catheter into an off-axis ventricle by passing the catheter through the bolt at the necessary angle.
A fifth objective of the present invention is to allow the doctor to replace a clogged drainage catheter without undue risk of infection: The infection risk of current devices is such that replacement catheters must be placed in a new hole drilled in the skull. The catheter passageway in the present invention is lined with a removable insert that incorporates all surfaces that might become contaminated. The insert is removed when the clogged catheter is removed thereby removing the contaminated surface and reestablishing a sterile field to allow the replacement of a clogged catheter without undue risk of infection.
A sixth-objective of the present invention is to reduce, the risk of infection by establishing a physical and chemical barrier to the ingress of bacteria between the surfaces of a bolt and a skull drill hole. Bacteria migration is discouraged in one version by attaching the bolt to the skull with a series of radial ribs that physically block the movement of bacteria down the drill hole. Bacteria survival is minimized by applying an antimicrobial agent tothe bolt portion that resides on and in the skull.
Another objective of the improvement in the present invention is to remove the possibility of introducing an error in the pressure reading in the 0-20 mmHg range due to the reluctance of the brain tissue surrounding the bladder to separate. The system is modified to cause that portion of brain tissue within which the bladder will reside to be dissected at the start of the procedure. The dissected section creates a pocket in brain tissue slightly larger than the volume of the bladder in its normal operating mode. A pocket can be created by a manual pump that overfills the bladder so it becomes somewhat larger than its normal operating volume. Alternatively, the pocket can be created by inserting a rod into the part of the brain that will be occupied by the bladder prior to inserting the bladder into the brain.
A corollary object of the improvement in the present invention is to provide a method of preconditioning the tissue by exposing the tissue to the dissecting means for a sufficient time for the tissue to adjust to the presence of the shape: that is, time enough for the tissues that are under load to tear and dissect until the dissection process is complete. The method requires that the overfilled bladder or the dissecting rod remain in place for up to a minute to assure that the brain tissue has fully responded to the dissection process.
In brief, the invention is carried out as follows. The invention addresses the limited ability of brain tissue to move by minimizing V in the expression P1V1=P2V2. V1 and V2 consist of Vnb=volume of non-bladder air and Vb=volume of bladder air. The volume change, V1xe2x88x92V2, required by the bladder to reflect a pressure change is reduced by minimizing Vnb. Vnb is minimized by attaching a pressure transducer close to the patient""s skull such as by directly coupling it to a bolt affixed to the skull. The invention greatly reduces the dead space in the long air line between the catheter and pressure transducer used in prior art. The reduction in dead space provides a number of benefits. First, it reduces brain trauma by reducing the air volume required by the bladder to reflect pressure change. Second, it improves three parameters that affect operating life. Operating life is defined as the period of time the bladder can operate before air lost by diffusion must be replaced. The three parameters are surface area, bladder stroke and the extent to which the bladder that can be filled with air and still function in brain tissue. The improvement in operating life is sufficient to allow the use of a manual pump in place of the presently used electromechanical pump.
The surface area through which water enters and gas is lost decreases as bladder volume decreases. The surface area of the bladder used in the present invention is ⅕ the surface area of the prior art bladder design now used. Water ingress and gas egress is therefore reduced by a factor of five.
Bladder stroke, i.e., the change in bladder volume, V1xe2x88x92V2 that accompanies a change in pressure is reduced as dead space is reduced. For example, in order to respond to a waveform with a 10-mmHg pulse pressure, the bladder stroke of the bolt mounted transducer is 1.4% vs. 6.0% for the partially inflated bladder of the prior art. The reduction in stroke volume allows the bladder to function when the air volume is more nearly depleted thereby extending the operating life of the bladder.
Minimizing dead space reduces bladder wall motion such that the bladder can function when the bladder to be more fully filled. The prior art bladder is filled to xcx9c25% of its potential. In contrast, the bladder of the bolt version and flexible catheter version can be filled to +80% and +50% respectively. The additional air extends life. The present invention further extends operating life by the use of two other elements.
The first element is the use of a bladder with more efficient collapsing characteristics. The second element is the use of a butyl rubber bladder. The prior art bladder is open at both ends and mounted on the catheter in a sleeve-like fashion. The bladder used in the present invention is closed at one end and open on the other. The open end is placed on the tip of the catheter. The tip-mounted bladder is able to collapse more completely than a sleeve mounted bladder and adds significantly to the operating life. A small diameter support element within the tip-mounted bladder allows the bladder to pass into the brain without folding over yet detracts little from the air volume available within the bladder.
The use of butyl reduces air loss by a factor of 5 vis a vis the prior art and limits water vapor ingress to the extent that the device can operate for the required monitoring period without risk of water condensate interfering with air movement between the bladder and transducer.
The five elements discussed make it possible for the bladder to operate for more than a nursing shift even though the gas volume is xc2xc to ⅕ that of the prior art. The present invention thereby makes it possible to substitute a manual pump for the electromechanical pump now used.
The invention covers three bolt-based systems and one flexible catheter system. Two bolt systems measure pressure in parenchymal brain tissue. A third system has the ability to measure ICP and to provide a passageway for the introduction of a drainage catheter into a ventricle. The three bolt systems are secured to the skull in a drill hole. The fourth system is a flexible catheter than can be used in conjunction with cranial operations where a flexible catheter is more easily placed than a bolt. The bolt systems will be discussed first.
The intraparenchymal system can be made in two versions. In one version, the catheter is integrated into a bolt and introduced into the brain as the bolt is placed in the drill hole. In another version, the catheter assembly and bolt are separate entities. The catheter is inserted into the bolt after the bolt is attached to the skull. The bladder in either case enters the brain to a depth of about 1 cm.
In the ventricular system, the catheter and bolt are integrated. The catheter enters the brain when the bolt is attached to the skull. The ventricular bolt provides a dedicated passageway through which the doctor can place a ventricular catheter if drainage is indicated at any time during the patient""s care. Thus, it is a dual function device that measures pressure with a minimally invasive pressure sensor and then provides a pathway through which a more invasive drainage catheter can be inserted if ICP is elevated. The system thereby allows the doctor to match the invasiveness of the procedure with the need of the patient. The passageway of the bolt is designed to allow the doctor to introduce a catheter at an angle to the axis of the bolt of about 15 degrees. The flexible angle of entry enables the doctor to enter a ventricle offset from the axis of the bolt. The ventricular bolt can be modified to provide a second port to allow insertion of a sensor into the brain that monitors a parameter other than pressure.
The bolt systems employ several features to reduce the risk of infection. In one version of the present invention, the screw conventionally used to attach the bolt to the skull is replaced with a series of radial ribs made of a deformable material. The bolt is tapped into the drill hole with a mallet. Interference between the ribs and the drill hole holds the bolt firmly to the skull. The ribs constitute a series of radial barriers that act as barriers to the migration of bacteria. In contrast, a conventional screw-in bolt provides a spiral pathway down which bacteria can migrate from the scalp to the brain if the monitoring period is prolonged. The invention further reduces the risk of infection by providing features in the product that apply an antimicrobial agent to the skull and to the drill hole. The features further act as a reservoir for the agent to provide continuing protection over time. The risk of infection is a serious issue as attested to by the fact that many doctors refuse to measure ICP due to risk of infection. The antimicrobial applicator and reservoir aspects are therefore important characteristics of the invention. The literature reports that the risk of infection of a bolt system is 4 times that of a non-bolt system wherein the catheter is tunneled beneath the scalp for a 3-5 cm after it exits the drill hole. The data concerning infection risk of a bolt system vs. a tunneled catheter underlines the merit of incorporating antimicrobial features into a bolt system.